In recent years, network systems such as LANs have been widely used in offices and factories, in which the LANs are built by connecting plural terminals including an embedded computer to one another. In such network systems, data is usually transmitted in packets. Headers of the packets include a destination (address) and a sender's address. Each terminal selectively processes only those packets intended for the terminal and performs operations based on commands or data included in the packets.
In many cases, the network systems as mentioned above have a function for simultaneously transmitting data (namely, packets) of the same contents to all terminals connected to the network. These packets are generally referred to as broadcast packets and the destinations are set such that they are the addresses of all the terminals connected to the network.
The systems using the broadcast packets do not need to generate packets in which destination addresses are separately set for all the terminals, so that this function is useful. However, it is known that the broadcast packets are massively generated in the network for some reason such as inconsistency of setting information in the network, program failure, and the like and this causes a serious network failure. Such a phenomenon is generally referred to as a broadcast storm.
In other words, when the broadcast packets are massively generated and the broadcast storm is generated, each terminal connected to the network receives a large amount of broadcast packets in a short time. Thus, a data processing unit such as CPU in each terminal spends most of the processing capacity in order to process the large amount of received broadcast packets, so that other processing requires a vary long time and the operation of the terminal may be suspended in the worst case.
If such a situation is caused, the terminal becomes inoperative until the broadcast storm is eliminated. Further, the inoperative terminal must be disconnected from the network and re-started. During this procedure, the operation and processing of the terminal is suspended. For example, when such a broadcast storm is generated in a network in a factory, since terminals include production equipment and manufacturing equipment, production lines may be stopped and great damage may be caused.
In order to solve the problems as mentioned above, logical disconnection of a computer system from a local area network (LAN) has been proposed, in which reception packets transmitted via the LAN are monitored in the computer system connected to the LAN and the computer system is logically disconnected from the LAN when a specific packet causing or capable of causing network failure is detected (refer to Patent Document 1, for example).
Also, disconnection of a specific terminal from a network cable has been proposed, in which a traffic monitoring device is connected to the network so as to detect the volume of traffic on the network and the specific terminal is disconnected from the network cable when the detected traffic volume exceeds a predetermined value (refer to Patent Document 2, for example).
Further, regarding techniques of the capability for processing received data, suspension of a transmission process at a node has been proposed, in which a medium is set to a busy status and the transmission process at the node about to transmit data is suspended when there is no room for reception in a buffer for temporarily storing received data (refer to Patent Document 3, for example).
Patent Document 1: Japanese Laid-Open Patent Application No. 4-334231
Patent Document 2: Japanese Laid-Open Patent Application No. 7-7515
Patent Document 3: Japanese Laid-Open Patent Application No. 2-92043
According to techniques disclosed in Patent Document 1, the terminal disconnects itself from the network when judging that received packets include a broadcast packet (specific packet) capable of causing a broadcast storm. Thus, information about the specific packet capable of causing the broadcast storm must be set in the terminal in advance. In other words, the specific packet capable of causing the broadcast storm is a known packet, so that when unknown packets are massively generated in the network, the terminal cannot recognize the unknown packets as the specific packet. In such a case, there is still a possibility that the terminal may become inoperative by receiving a large amount of unknown packets.
On the other hand, according to techniques disclosed in Patent Document 2, it is not necessary to set information about the specific packet in the terminal in advance, since a data volume (packet volume) flowing on the network is monitored and the generation of abnormality such as the broadcast storm is detected. Thus, even when unknown packets are massively generated, it is possible to disconnect a specific terminal from the network. However, the monitoring of the traffic volume and the disconnection of the terminal are performed using the traffic monitoring device, so that the traffic monitoring device must be connected to the network separately from each terminal. Further, a terminal to be disconnected when the traffic volume is increased must be set in advance in the traffic monitoring device.
In this manner, according to the techniques disclosed in Patent Document 2, the specific terminal is disconnected from the network in order to control the traffic volume in the entire network system and the terminal to be disconnected is determined by other terminal (traffic monitoring device) connected to the network. In other words, the terminal does not disconnect itself from the network based on the packet volume received by the terminal.
Moreover, according to techniques disclosed in Patent Document 3, the medium is set to a busy status and transmission is delayed when there is no available capacity in the buffer for temporarily storing received data to be processed, so that all received packets are assumed to be processed. In other words, the terminal is not disconnected and processing is suspended when the volume of the received packets is too large. Thus, when temporarily receiving a large amount of packets as in the case of the broadcast storm, the reception of the large amount of packets is merely delayed and the packets are processed afterward. Thus, it is impossible to solve the problem of being unable to process packets to be processed other than unnecessary broadcast packets.